Pressure measuring apparatuses are often applied in process measurements technology, in order to measure the pressure of process media, which can be liquids, gases or vapors. Such measuring devices or pressure sensors essentially comprise a pressure measuring cell or a pressure transducer, which generally has a platform and an elastic membrane. The platform and the membrane are, depending on embodiment, composed of ceramic, or at least the membrane is at least partially composed of metal. Possible combinations of membrane and platform are, for example, ceramic/ceramic, metal/ceramic or metal/metal, with dielectric elements composed of ceramic or glass. The method for soldering metal onto ceramic is known, and can, for example, be implemented for the combination, titanium/aluminum oxide ceramic (or with special alloys, such as Kovar, in the place of the titanium). A dielectric ceramic of aluminum titanate with very low coefficient of expansion can, for example, also be combined with a nickel alloy, Invar, or a silicon carbide ceramic in the soldering method. Another embodiment has a stainless steel membrane welded onto a stainless steel platform. In the platform, a space for an insulating element for the measuring electrodes is preferably provided in the case of this embodiment. On the platform, a shallow cavity is most often provided, which is also referred to as a membrane bed, and is covered by the membrane. In measurement operation, the membrane is contacted with the pressure of the process medium, and the deformation or the mechanical stress in the elastic membrane, which, for example, is capacitively or piezo resistively ascertained, is a measure for the pressure (see e.g. Offenlegungsschrift DE 39 01 492).
In the case of ceramic cells of such pressure transmitters, crack formation in the membrane can be a cause for a failure of the cell. The thickness of such membranes is dependent on the measuring range of the pressure, and most often lies, for instance, between 0.1 and 2 mm. It is known that, in contrast to metals, ceramic materials, due to their brittleness and the lack of plastic deformability resulting therefrom, can break very quickly when the mechanical stress reaches the ultimate tensile strength. The cause for a breaking of the membrane can be microscopic cracks in the ceramic, which, under a load, have reached an over-critical growth. In the case of some manufacturing steps, e.g. working the material (for instance machining the material) while it is hard, cracks can be caused, which, however, cannot be recognized due to their small size. Another cause for critical crack growth can be a local overloading of the components due to an unfavorable combination of thermal gradients during a temperature shock in the case of a simultaneous pressure or vacuum loading. The measuring cells can thus be mechanically and thermally overloaded in applications, and, react thereto with crack growth. In the extreme case, cracks are recognized through leakage associated therewith, whereby to some degree complex maintenance measures are required. In the case of metal membranes, it is furthermore possible that, due to excessive pressure, a plastic deformation will occur, which leads to a marked measurement deviation. In the state of the art, for detecting membrane breakage, the application of, for example, a sacrificial membrane is provided, which is damaged by a corrosive attack or other critical events. The failure of the sacrificial cell serves as a sign that the measuring cell is also damaged or is shortly about to become damaged.